The contact measurement usually used in cold strip mills is only possible in the hot strip field at the expense of substantial outlay on maintenance, because of the high strip temperature of from about 1000.degree. C. Contact measurement on the end faces of a coil being formed in a coiler is also not possible. It is therefore difficult, if not completely impossible, to coil strip in such a way that in the coil every turn lies exactly over the preceding one so that flat end faces are obtained. And even in cold strip mills efforts are made to avoid contact measurement, since the mechanical measuring elements have only a limited life.
Strip flatness is therefore preferably measured without contact. For example, it is known to measure departures from flatness by means of spots of light projected on to the strip. The position in space of the light spot produced on the surface of the strip, preferably by means of a laser beam, is detected using a range finder.
The two plane position coordinates of a particular point on the surface are known from the position of the scanning or illuminating beam relative to the surface of the strip. The height coordinate of the point on the surface which is currently being measured is detected by a position-sensitive sensor. The position of the image point on the sensor varies simultaneously with the height coordinate.
Using a large number of sources of radiation and sensors a flatness image can be built up over the whole width of the strip which is made up from the results of measurement of the spots of light projected on the strip at particular distances apart. Nevertheless, in this method the regions between the points of light are not detected an in the case of continuous strip from strip-shaped measurement gaps in which the flatness is not determined. Moreover, this can result in measurement errors, for example through wobbling or flattering of the strip being detected by the measuring method as uneveness of the strip.
In the automobile industry it is known to measure relatively small surfaces using the moire technique. In this method an interference pattern is produced on the surface of the object by means of a light source. The interference pattern is detected using a CCD (charge-coupled device) camera. The camera is arranged so that an angle is formed between the light source, the surface and the camera. By the use of a reference grid in the image plane a so-called miore effect is obtained by superposition of the detected pattern and the refernce pattern. The height differences can be determined quantitatively from the moire lines.
The miore technique provides more accurate measurement result than measurement using spots of light and moreover it covers substantially the whole of the surface to be measured and avoids the measurements gaps mentioned above. However, its use involves problems, particularly in a hot strip mill.
To determine the height differences of the rolled strip a complicated conversion of the pattern detected by the camera is necessary. The height differences pictured as miore lines cannot be converted into quantitative measured values in real time.
But in a rolling mill train rapid results are precisely what is required from measurements, since otherwise it is hardly possible to use the measurement for direct adjustmente of the rolling parameters so as to improve the flatness of the continuous strip. Moreover for industrial application the fine interference patterns are lacking in contrast and intensity.
In the case of conventional metal strip mill trains, in which the strip flatness is measured by one of the above-mentioned methods, departures from flatness are not measured from the cooling line, and this can result in considerable loss in quality.